Abstract

Patient-specific measurements are typically used to validate the dosimetry of intensity-modulated radiotherapy(IMRT). To evaluate the dosimetric performance over time of our IMRT process, we have used statistical process control (SPC) concepts to analyze the measurements from 330 head and neck (H&N) treatment plans. The objectives of the present work are to: (i) Review the dosimetric measurements of a large series of consecutive head and neck treatment plans to better understand appropriate dosimetric tolerances; (ii) analyze the results with SPC to develop action levels for measured discrepancies; (iii) develop estimates for the number of measurements that are required to describe IMRTdosimetry in the clinical setting; and (iv) evaluate with SPC a new beam model in our planning system. H&N IMRT cases were planned with the PINNACLE3treatment planning system versions 6.2b or 7.6c (Philips Medical Systems, Madison, WI) and treated on Varian (Palo Alto, CA) or Elekta (Crawley, UK) linacs. As part of regular quality assurance, plans were recalculated on a -diam cylindrical phantom, and ion chamber measurements were made in high-dose volumes (the PTV with highest dose) and in low-dose volumes (spinal cord organ-at-risk, OR). Differences between the planned and measured doses were recorded as a percentage of the planned dose. Differences were stable over time. Measurements with PINNACLE3 6.2b and Varian linacs showed a mean difference of 0.6% for PTVs (, range, to 6.6%), while OR measurements showed a larger systematic discrepancy (mean , range to 16.3%) that was due to well-known limitations of the MLC model in the earlier version of the planning system. Measurements with PINNACLE3 7.6c and Varian linacs demonstrated a mean difference of 0.2% for PTVs (, range, , to 5.0%) and for ORs (range to 4.4%). The capability index (ratio of specification range to range of the data) was 1.3 for the PTV data, indicating that almost all measurements were within . We have used SPC tools to evaluate a new beam model in our planning system to produce a systematic difference of for PTVs and 0.4% for ORs, although the number of measurements is smaller . Analysis of this large series of H&N IMRT measurements demonstrated that our IMRTdosimetry was stable over time and within accepted tolerances. These data provide useful information for assessing alterations to beam models in the planning system. IMRT is enhanced by the addition of statistical process control to traditional quality control procedures.

Received 18 December 2007Revised 22 July 2008Accepted 23 July 2008Published online 11 September 2008

Acknowledgments:

The authors gratefully acknowledge Dr. Mohammed Islam and our team of Physics Associates in the Physics Department at Princess Margaret Hospital who have carried out our IMRT dosimetric measurements for several years, and Dr. Claire McCann and Dr. Andrea McNiven who assisted in dose computation for continuous beam model adjustments. We thank the referees for their careful reading of the manuscript, and their many insightful comments that have improved this paper.